Genetically engineered plants were introduced to the commercial market in 1996 and are now grown worldwide.

United States: 154 million acres of soybeans, corn, cotton, canola and other crops

Argentina: 52 million acres of soybeans, corn and cotton

Brazil: 39 million acres of soybeans, corn and cotton

India: 19 million acres of cotton

Canada: 19 million acres of canola, corn, soybeans and sugar beets

SOURCE: International Service for the Acquisition of Agri-biotech Applications, 2008

If the titans of agribusiness are right, the world is on the verge of a major breakthrough in the way food is grown. But if history is any indication, genetically modified crops will need to overcome a lot of skepticism to spark a consumer revolution.

That uncertainty is fueled by the mixed record of current bioengineered crops — mainly soybeans, corn, canola — in meeting lofty targets set by backers of high-tech seeds. Vitamin-enhanced foods remain out of reach, it’s unclear how much biotechnology has boosted plant production, and a recent study said genetically engineered plants have increased usage of herbicides.

One of biotechnology’s leading advocates will hold a presentation at the 176th annual meeting of the American Association for the Advancement of Science, which will start Thursday at the San Diego Convention Center and other venues. About 8,000 people from more than 50 countries are expected for the five-day affair, making it the largest general-science conference in the nation.

“We are just scratching the surface of the science that is now being applied to improve crops,” said Robert Fraley, chief technology officer for Monsanto Co., an agricultural giant based in St. Louis. “I think we will see tremendous gains in yields and profitability for farmers, and health and nutrition for consumers.”

A handful of other events will focus on nourishing a global population of 6.8 billion and growing.

“Agriculture must produce more food worldwide in the next 50 years than has been produced in the past 10,000,” Fraley said.

He will talk about innovations such as drought-tolerant and higher-yielding plants that are in the product pipeline. He will likely face a sympathetic crowd: San Diego is home to a small but high-profile cluster of plant geneticists who do basic research on plant genomes, which biotech companies need to develop commercial varieties.

Monsanto is seeking regulatory approval for what may be the first genetically engineered crops developed directly for the consumer. The company’s scientists in California have manipulated the soybean genome to produce omega-3 fatty acids, the kind that nutritionists tout as heart-healthy.

It could be pivotal in generating public acceptance of biotech, or “transgenic,” crops. To date, farmers and seed companies have reaped benefits from biotechnology while consumers have been left to wonder what’s in it for them and whether genetic engineering has really made the world a better place.

Scientists bioengineer crops by snipping carefully selected genes from plants, bacteria or viruses and inserting them into the cells of plants that are commercially significant, such as corn, alfalfa and cotton. These “transgenes” produce unusual effects in their host plants.

In some cases, they make host plants produce a compound that’s toxic to insects. Researchers also have created crops able to withstand herbicides that typically would kill them. This tweak allows growers to douse weeds without harming their harvest.

Pest- and herbicide-tolerant varieties have been wildly popular among farmers since their introduction in 1996. They’re now grown on more than 300 million acres annually worldwide.

Some biotech backers said the public soon will see plants that need less water and fertilizer, allowing them to flourish in arid regions and places where the soil is too poor for conventional crops.

Locally, about 15 scientists have formed the San Diego Center for Molecular Agriculture to help advance the movement. Their work centers on finding the genes that trigger certain plant responses and not on creating commercially viable products, said Maarten Chrispeels, an emeritus professor of biology at the University of California San Diego.

“These days, there is much to do about getting plants to extract nutrients from the soil more efficiently,” he said.

It’s an alluring goal for farmers, humanitarian groups and companies that patent various plant traits.

For years, they have touted the potential of genetically engineered rice that’s rich in beta-carotene — a precursor to vitamin A — as a way to decrease malnutrition in Asia. This “golden rice,” named for its distinctive yellow color, has drawn international attention for more than a decade but still hasn’t been released to the public.

Scientists have found it time-consuming to get the vitamin A trait into local rice varieties, and it takes years to gain government clearance for biotech crops. Regulations are abundant because many people worldwide are uneasy about the potential for lab-formed plants to create health and environmental problems that don’t show up until they have spread widely.

Chrispeels said fears about genetically engineered food have calmed considerably in the United States during the past few years, despite continued angst in Japan and Europe.

“We have had biotech crops for 15 years now and they are not going away, even if the Europeans don’t like them,” he said.

His enthusiasm aside, Chris-peels isn’t convinced that companies will produce many crops offering benefits to consumers.

“The emphasis will remain the way it was — things that benefit production,” he said. “There are so many ways of getting a healthy diet, you don’t actually need transgenics.”

Other scientists are much more critical of the technology, saying it has failed to deliver on claims that it could feed the world and reduce the ecological damage done by farming.

“People who have been listening to these promises for a long time would have reason to be skeptical,” said Margaret Mellon, director of the food and environment program at the Union of Concerned Scientists in Washington, D.C.

A study published in April by Mellon’s group concluded that worldwide food production exceeds demand, calling into question the need for more biotech crops. It said gains in agricultural productivity are largely due to improvements in conventional breeding and other farming techniques — not biotechnology.

In November, The Organic Center in Boulder, Colo., determined that the first 13 years of biotech crops resulted in 318 million more pounds of pesticides used in the United States compared with what would have been sprayed on a similar amount of conventional crops.

The big jumps occurred in 2007 and 2008, said study author Charles Benbrook, a critic of bioengineered plants. He said weeds are adapting to glyphosate, the main chemical sprayed on herbicide-tolerant crops, so farmers are using more.

Monsanto officials point to reports that tout an overall environmental benefit from biotechnology. For instance, farmers can reduce fuel use by spraying chemicals less often and tilling fields fewer times, two common advantages with biotech plants.

Benbrook said some genetically engineered traits have worked well, but he remains unconvinced that biotech is the best bet for solving the world’s food and farming problems.

“Farmers can grow plenty of vitamin A by growing squash and carrots,” he said.